This invention relates to methods for determining proteins in biological samples using immobilized specific receptors for the proteins. In particular it relates to the reduction of nonspecific binding of radioiodine labelled protein hormones (tracers) in such methods.
Nonspecific binding is defined as the radioactivity found in the solid phase of a heterogenous assay conducted without antibody to the analyte in question. The interactions which are responsible for this phenomenon have not been fully elucidated. It is desirable to reduce the non-specific binding to as low a level as possible because the assay sensitivity and accuracy are otherwise adversely affected.
Protein harmones such as prolactin are relatively low molecular weight substances, on the order of 8,000 to 80,000 MW. They are well-known and largely fully characterized. Examples include, in addition to prolactin, insulin, gonadotropin, growth hormone, ACTH, thyrotropin and parahormone. Prolactin is susceptible to changes in molecular aggregation or polymerization of the hormone monomer units during purification and storage. For example see Garnier et al., "J. Endocrin. and Metabolism" 47(6):1273-1281 (1978). It has also been observed by Fang et al. in "Clin. Chem." 24(6):941-943 (1978) that radioiodinated prolactin compositions containing high proportions of "small" or monomeric prolactin (MW 23,000) improve the prolactin assay sensitivity and consistency over compositions containing larger proportions of "big" prolactin (MW 170,000).